Magnetic Survey

Geological and buried manmade features or objects can cause local variations in the earth’s magnetic field which can be measured with a magnetometer. Detailed magnetic survey can be used to effectively define areas of past human activity. It is a quick, cost effective technique which has become the industry standard for archaeological surveys.

Buried Objects

Objects that have been heated have their magnetic properties markedly changed. These are called 'thermoremanent' effects. These effects can be very large and strong such as an igneous dyke, through to a horse shoe nail in the top soil causing a 'ferrous' spike. Features with thermoremanent magnetisation can be easily found with gradiometers so that objects such as brick walls, foundations, steel or clay pipes, hearths, kilns and ferrous artifacts associated with archaeology will stand out from the background magnetic levels.

Cut Features

In addition to thermoremanent effects more subtle changes in magnetisation can be detected. Silting and deliberate infilling of ditches and pits with magnetically enhanced soil creates a relative contrast against the much lower levels of magnetism within the subsoil into which the feature is cut. Material such as subsoil and non-magnetic bedrock used to create former earthworks and walls may be mapped as areas of lower enhancement compared to surrounding soils.

Factors affecting the magnetic survey may include soil type, local geology, previous human activity, disturbance from modern services etc.

The new SUMO cart system

The new SUMO cart system utilises four Bartington gradiometers combined with a Trimble RTK GPS. With improved resolution and each data point positioned with the survey-grade GPS, the system represents a significant step forward in data acquisition.